The long-term goal of this research is to study the dynamics of neurotransmitter signaling in the central nervous system of Drosophila melanogaster, the fruit fly. The specific goal of this project is to develop a microelectrode method to measure the real-time release and clearance of serotonin in an intact fruit fly nervous system. The fruit fly is a popular model system for biologists because of the ease of making genetic mutations and its homology with higher order organisms, including similar neurotransmitter systems. The small size of the fly central nervous system (8 nL) has precluded real-time studies of neurotransmitter release and uptake. This study has implications for human health because serotonin is an important neurotransmitter whose signaling is implicated in mental illnesses, including depression and drug abuse. However, the mechanisms for regulation of serotonin concentrations are not well understood. The specific aims are: 1. To characterize real-time, endogenous serotonin release and uptake in individual Drosophila larva. Genetically modified flies with a blue-light sensitive channel will be used to specifically elicit serotonin release. This technique will be used to test hypotheses about the effects of cocaine administration on serotonin transporter activity and the effects of genetic manipulations on neurotransmitter release. 2. To compare the regulation of serotonergic and dopaminergic signaling in Drosophila. Repeated stimulations will be used to assess the importance of synthesis and recycling in maintaining release. 3. Development of a rapid assay for serotonin transporter activity in Drosophila. Small amounts of serotonin will be pressure ejected into the nerve cord and then detected at the microelectrode. This technique will allow tests of hypotheses about the effect of pharmacological agents or mutations of the serotonin transporter on serotonin signaling. These experiments will result in assays for the fly that will be valuable for screening genetic factors that regulate neurotransmitter levels, resulting in a better fundamental understanding of the time course of neurotransmission.